EGU General Assembly 2021
© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

Nitrogen fixation in a diazotrophic post-bloom situation in the Baltic Sea

Christian Reeder1 and Carolin Löscher2
Christian Reeder and Carolin Löscher
  • 1Southern University of Denmark, Biology, Odense M, Denmark (
  • 2Southern University of Denmark, Biology, Odense M, Denmark (

The Baltic Sea is characterised as a semi-enclosed brackish Sea that has experienced increased eutrophication, hypoxia, and increased temperature over the last ~100 years making Baltic Sea one of the most severely impacted oceanic environment by climate change. Biological fixation of dinitrogen gas (N2) is an essential process to make atmospheric N2 available for marine life. This process is carried out by specialised organisms called diazotrophs and is catalysed by the energetic-consuming enzyme nitrogenase. Nitrogenases exist in three subtypes depending on their metal cofactors, (1) the most common molybdenum-dependent (Nif), (2) the vanadium-dependent (Vnf) and (3) the Iron-Iron-dependent nitrogenase (Anf). To date, the effect of climate change on those three enzyme subtypes and their potential role a future ocean is yet to be explored. The predicted ongoing oxygen loss in the ocean may limit Mo's availability and trigger a shift from the abundant Nif-type nitrogenase to Vnf or Anf and, therefore, a potential shift in the diazotrophic community. This study explored the climate change-related pressures on N2 fixation and the diazotrophic community based on nifH and vnf/anfD amplicons. At the time of sampling, we found a post-bloom high-nutrient low-chlorophyll situation. Cyanobacterial groups, Nodularia and UCYN-A, dominated the diazotrophic community and showed a horizontal where UCYN-A were the dominant fixers at 20 m. Based on alternative nitrogenases amplicons, Rhodopseudomonas was the dominating microbe in the surface water. This paper presents the first hint of active nitrogenases in surface water and further establish UCYN-A as a significant player in Baltic Sea primary production.

How to cite: Reeder, C. and Löscher, C.: Nitrogen fixation in a diazotrophic post-bloom situation in the Baltic Sea, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-911,, 2021.


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